Gas separation membrane

ABSTRACT

A method of fabricating a gas separation membrane includes providing a coextruded multilayer film that includes a first polymer layer formed of a first polymer material and a second polymer layer formed of a second polymer material, the first polymer material having a first gas permeability. The coextruded multilayer film is axially oriented such that the second polymer layer has a second gas permeability that is greater than the first gas permeability.

RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 61/640,758, filed May 1, 2012, the entirety of which is incorporatedherein by reference.

GOVERNMENT FUNDING

This invention was made with government support under Grant Nos.CON500535, and RES501043 awarded by The National Institutes of Health.The United States government has certain rights to the invention.

FIELD OF THE INVENTION

The present invention relates to membranes and, in particular, relatesto a thin, coextruded membrane or film with high flux and highselectivity.

BACKGROUND

It is known that polymer and polymer-based membranes may be used in awide variety of fluid separation processes, such as gas separation,desalination, removal of pathogens or other substances from liquids,such as water, and other applications requiring selective permeation,such as controlled atmosphere food packaging. In gas separations, forexample, membranes can be used to separate air into oxygen-rich andnitrogen-rich streams. Gas separation membranes can also be used toremove carbon dioxide and other impurities from natural gas as well asselective removal of hydrogen from a wide variety of process streamsimportant in the chemical, petrochemical, and other industries.

For many applications of membranes, including those mentioned above, themembranes are only commercially viable if they can be made very thin, inmany cases on the order of less than about 10 μm or so in thickness. Asmembranes become thinner, the probability of developingselectivity-destroying pinhole defects in the membrane becomes higher.

Separation membranes can be prepared on large scale from solutions ofpolymers in a suitable solvent by methods widely known in the art. Thepolymers used in these applications, including, polysulfones,polyimides, poly(dimethyl siloxanes), polyethers, poly(vinylidenefluoride) and related materials, are typically only soluble in organicsolvents. The solvents constitute the dominant mass in membraneprocessing and must be removed following membrane formation. Thesesolvents, can be flammable and toxic. Additionally, these solvents areexpensive not only to purchase but also to dispose of at the end ofmembrane processing. In many cases, solvent costs, including initialsolvent purchases, solvent handling equipment, and solvent disposalequipment and processes are significant costs in the manufacturing ofmembranes for fluid separations. Therefore, there is a need in themembrane separation field to have methods to prepare membranes viasolventless processes.

SUMMARY

Embodiments described herein relate to a gas separation membrane thatincludes an axially oriented, coextruded multilayer film that has aCO₂/O₂ selectivity of at least about 4 and a flux of at least about 20GPU. The axially oriented, coextruded multilayer film can include atleast one axially oriented, coextruded first polymer layer of a firstpolymer material and at least one axially oriented, coextruded secondpolymer layer of a second polymer material. The at least one axiallyoriented, coextruded first polymer layer can have a first gaspermeability (P₁) prior to axial orientation and a second gaspermeability (P₂) after axially orientation less than or equal to thefirst permeability (P₁). The at least one axially oriented, coextrudedsecond polymer layer can have a first gas permeability (P_(1a)) prior toaxial orientation and a second gas permeability (P_(2a)) after axialorientation that is substantially greater than first permeability(P_(1a)) and the second permeability (P₂). In some embodiments, theaxially oriented, coextruded multilayer film can include a plurality ofaxially oriented, coextruded alternating first polymer layers and secondpolymer layers.

In some embodiments, the axially oriented, coextruded first polymerlayer has a first thickness and the combined thicknesses of all theaxially oriented, coextruded first polymer layers of the axiallyoriented, coextruded multilayer film can be less than about 1 μm.

In other embodiments, the at least one axially oriented, coextrudedfirst polymer layer can have a CO₂/O₂ selectivity of at least about 4and a flux of at least about 30 GPU.

In still other embodiments, the first polymer material can be apoly(ether block amide. The poly(ether block amide) can include fromabout 15% to about 80% of a polyether by molecular weight. The secondpolymer material can include a polypropylene. The polypropylene canfurther include CaCO₃ or a beta-nucleation agent.

Other embodiments described herein relate to a method of fabricating agas separation membrane. The method includes coextruding a first polymermaterial and a second polymer material to form a coextruded multilayerfilm that includes at least one coextruded first polymer layer and atleast one coextruded second polymer layer. The at least one coextrudedfirst polymer layer can have a first permeability (P₁) and a CO₂/O₂selectivity of at least about 4. The coextruded multilayer film is thenaxially oriented to provide an axially oriented, coextruded multilayerfilm that has a CO₂/O₂ selectivity of at least about 4 and a flux of atleast about 20 GPU. The at least one axially oriented, coextruded firstpolymer layer can have a second permeability (P₂) after axialorientation that is less than or equal to the first permeability (P₁).The at least one axially oriented, coextruded second polymer layer canhave a first permeability (P_(1a)) prior to axial orientation and asecond permeability (P_(2a)) after axial orientation that issubstantially greater than first permeability (P_(1a)) and the secondpermeability (P₂). The method of forming the axially oriented,coextruded multilayer film can be a solventless and/or substantiallysolventless or a solvent-free process.

In some embodiments, the multilayer film can be axially oriented at atemperature that is below the melting temperature (T_(m)) of the secondpolymer material. In other embodiments, the axial orientation comprisesuniaxial stretching.

In still other embodiments, the axially oriented, coextruded multilayerfilm can include a plurality of axially oriented, coextruded alternatingfirst polymer layers and second polymer layers. The axially oriented,coextruded first polymer layer can have a first thickness, and thecombined thicknesses of all the axially oriented, coextruded firstpolymer layers of the axially oriented, coextruded multilayer film canbe less than about 1 μm.

In some embodiments, the coextruded multilayer film is axially stretchedabout 100% to about 400%. In other embodiments, the multilayer film isheat treated at a temperature and for a time effective to increase thegas permeability of the at least one axially oriented, coextruded firstpolymer layer. In still other embodiments, the axially orienting the atleast one first polymer layer can cause strain-induced crystallizationin the at least one first polymer layer and heat treating the axiallyoriented coextruded multilayer film at least partially reversescrystallization in the at least one axially oriented coextruded firstpolymer layer to increase the gas permeability of the at least oneaxially oriented coextruded first polymer layer.

The multilayer film described herein can be used in a wide range offluid separation processes, such as gas separation, desalination,removal of pathogens or other substances from liquids, such as water,and other applications requiring selective permeation, such ascontrolled atmosphere food packaging.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic view of a multilayer film in accordancewith an aspect of the application.

FIG. 2 is a schematic illustration of a layer-multiplying coextrusionprocess for forced-assembly of polymer nanolayers in accordance with anaspect of the application.

FIG. 3 is a schematic illustration of a layer-multiplying coextrusiondie for forced-assembly of polymer nanolayers in accordance with anotheraspect of the application.

FIG. 4 is a schematic illustration of a method of forming an axiallyoxiented multilayer film that is subsequently heat treated.

FIG. 5 is a graph illustrating gas permeability relative to PTMO contentin PEBAX film layers.

FIG. 6 illustrates images of unoriented and axially oriented PEBAX filmlayers.

FIG. 7 is a graph illustrating oxygen permeability of axially orientedPEBAX film layers

FIG. 8 is a graph illustrating oxygen permeability of elasticallyrecovered PEBAX film layers.

FIG. 9 is a graph illustrating oxygen permeability of annealed PEBAXfilm layers.

FIG. 10 illustrates WAXS images of PEBAX film layers before and afterannealing.

FIG. 11 is a graph illustrating oxygen flux of uniaxially orientedPEBAX/PP+CO₃ multilayered films.

FIG. 12 is a graph illustrating the effect of temperature on axiallyorienting PEBAX/PP+CO₃ multilayered films.

FIG. 13 is a graph illustrating oxygen flux of annealed PEBAX/PP+CO₃multilayered films.

FIG. 14 is a graph illustrating oxygen flux of biaxially orientedPEBAX/PP+QQ multilayered films.

FIG. 15 is a graph illustrating oxygen flux of annealed PEBAX/PP+QQmultilayered films.

FIG. 16 is a graph illustrating oxygen flux of biaxially orientedPEBAX/β-PP multilayered films.

FIG. 17 is a graph illustrating oxygen flux of annealed PEBAX/β-PPmultilayered films following biaxial orientation.

FIG. 18 is a graph illustrating oxygen flux of annealed PEBAX/β-PPmultilayered films prior to biaxial orientation.

DETAILED DESCRIPTION

Embodiments described herein relate to membranes, such as gas separationmembranes, and, in particular, relate to thin, coextruded, multilayermembranes with high flux (e.g., a flux of at least about 20 GPU) andhigh CO₂/O₂ selectivity (e.g., a CO₂/O₂ selectivity of at least about4). The membrane can include an axially oriented, coextruded multilayerfilm that has a CO₂/O₂ selectivity of at least about 4 and a flux of atleast about 20 GPU. The axially oriented, coextruded multilayer film caninclude at least one axially oriented, coextruded first polymerselective layer of a first polymer material and at least one axiallyoriented, coextruded second polymer support layer of a second polymermaterial. The at least one axially oriented, coextruded first polymerlayer can have a first gas permeability (P₁) prior to axial orientationand a second gas permeability (P₂) less than or equal to the first gaspermeability (P₁) after axial orientation. The at least one axialoriented, coextruded second polymer layer can have a first gaspermeability (P_(1a)) prior to axial orientation and a second gaspermeability (P_(2a)) after axial orientation that is substantiallygreater than first permeability (P_(1a)) and the second permeability(P₂). In some embodiments, the axially oriented, coextruded multilayerfilm can include a plurality of axially oriented, coextruded alternatingfirst polymer layers and second polymer layers.

The at least one axially oriented, coextruded second support layer canbe sufficiently porous and/or permeable such that the flux andselectivity of the at least one axially oriented, coextruded firstpolymer selective layer defines the flux and selectivity of the axiallyoriented, coextruded multilayer film. The at least one axially oriented,coextruded first polymer layer can have a flux of, for example, at leastabout 10 gas permeation units (GPU), at least about 20 GPU, or at leastabout 30 GPU, and the axially oriented, coextruded multilayer film canhave a flux of at least about 10 GPU, at least about 20 GPU, and atleast about 30 GPU. The at least one axially oriented, coextruded firstpolymer layer can have a CO₂/O₂ selectivity of, for example, at leastabout 4, at least about 5, at least about 6, at least about 7, at leastabout 8, at least about 9, or at least about 10, and the axiallyoriented, coextruded multilayer film can have a selectivity of at leastabout 4, at least about 5, at least about 6, at least about 7, at leastabout 8, at least about 9, or at least about 10. By way of example, theaxially oriented, coextruded multilayer film can have a flux of at leastabout 10 GPU, at least about 20 GPU, or at least about 30 GPU and aCO₂/O₂ selectivity of at least about 4, for example, about 6 to about10.

The axially oriented, coextruded multilayer film can be formed in a twostep process. In the first step, a first polymer material and a secondpolymer material are coextruded to form at least one first polymer layerand at least one second polymer layer of the first and second polymermaterials, respectively. In the second step, the multilayer film isaxially oriented or stretched in at least one direction to provide anaxially oriented, coextruded multilayer film that has a CO₂/O₂selectivity of at least about 4 and a flux of at least about 20 GPU. Theat least one axially oriented, coextruded first polymer layer can have afirst permeability (P₁) prior to axial orientation and a secondpermeability (P₂) after axial orientation less than or equal to thefirst permeability (P₁). The at least one axially oriented, coextrudedsecond polymer layer can have a first permeability (P_(1a)) prior toaxial orientation and a second permeability (P_(2a)) after axialorientation that is substantially greater than first permeability(P_(1a)) and the second permeability (P₂). The method of forming theaxially oriented, coextruded multilayer film can be a solventless and/orsubstantially solventless or solvent free process, i.e., the multilayerfilm is formed and processed substantially free of solvents or in someapplications entirely without the use of solvents.

FIG. 1 is a schematic illustration of an axially oriented, coextrudedmultilayer film 10 in accordance with an embodiment described herein.The axially oriented, coextruded multilayer film 10 comprisesalternating axially oriented, coextruded first polymer selective layers12 and second polymer support layers 14. Although multiple first polymerselective layers 12 and multiple second polymer support layers 14 areillustrated, it will be appreciated that the axially oriented,coextruded multilayer film 10 may only include one first polymerselective layer and/or only one second polymer support layer.

In some embodiments, e.g., produce packaging, the axially oriented,coextruded multilayer film 10 can have a high CO₂/O₂ selectivity, a highgas flux through the film, and structural stability. In someembodiments, the axially oriented, coextruded multilayer film can have aCO₂/O₂ selectivity of at least about 4, for example, about 6 to about 10and a gas flux of at least about 20 GPU, for example, at least about 30GPU.

The first polymer selective layers 12 and second polymer support layers14 can be made of or formed from materials that upon coextrusion andaxial orientation provide the axially oriented, coextruded multilayerfilm with such a high CO₂/O₂ selectivity (e.g., at least about 4) and ahigh flux (e.g., at least about 20 GPU) as well as low temperatureflexibility, low water sensitivity, and/or chemical resistance.

The first polymer selective layer can be formed from a first polymermaterial that can be readily coextruded with a second polymer material,axially oriented, e.g., uniaxially or biaxially stretched or drawn, andwhen coextruded and axially oriented form a layer or plurality of layersthat has a high CO₂/O₂ selectivity (e.g., at least about 4) and a highflux (e.g., at least about 20 GPU). The first polymer material used toform the first polymer layers 12 can, for example, include anythermoplastic or thermoformable polymer material that can be readilycoextruded and axially oriented, e.g., uniaxially or biaxially stretchedor drawn and when coextruded and axially oriented form a layer orplurality of layers that has a high CO₂/O₂ selectivity (e.g., at leastabout 4) and a high flux (e.g., at least about 20 GPU).

Examples of polymers that can be used as the first polymer material arethermoplastic elastomers, such as polyethylene, polyethylene oxide(PEO), polycaprolactone (PCL), polyether based materials, such aspolytetramethylene oxide (PTMO), and poly(ether block amide) (e.g.,PEBAX, which is commercially available from Arkema, Inc.).

In some embodiments, the poly(ether block amide) can have the generalformula:

where PA is a polyamide and PE is a polyether. The composition ratio ofPE/PA in the copolymer can vary from about 15/85 to about 85/15. Thepolyamide component can include any conventional polyamides, such asnylon 6, nylon 66, nylon 11, and nylon 12. The polyether component canbe selected from polyoxyethylene, polyoxypropylene, andpolytetramethylene oxide. In some embodiments, the PTMO percentage canvary from about 15% to about 80% by molecular weight. In someembodiments the PTMO percentage is at least about 60% by molecularweight. Although specific materials for the first polymer material areenumerated, it will be appreciated that alternative polymers,copolymers, and combinations thereof may be used that meet theaforementioned performance criterion, e.g., upon coextrusion and axialorientation provide an at least one first polymer layer having a CO₂/O₂selectivity of at least 4 and a flux of at least about 20 for themultilayer film 10.

The second polymer support layers 14 can be formed of a second polymermaterial that upon coextrusion and axial orientation with the firstpolymer material forms microporous second polymer support layers thatthat have a permeability and flux substantially greater than the firstpolymer selective layers (e.g., at least about 2 times greater, at leastabout 4 times greater, at least about 5 times greater, or at least about10 times greater) and that can be essentially non-selective to ornegligible to the skeleton of the flow of CO₂/O₂ through the secondpolymer support layers. The second polymer material used to form thesecond polymer layers can include thermoplastic or thermoformablepolymers that are immiscible or partially miscible with the firstpolymer material upon coextrusion. To this end, the second polymermaterial can include any thermoplastic or thermoformable polymermaterial with a viscosity that is substantially similar to the viscosityof the first polymer material. Furthermore, the first polymer materialand second polymer material may be selected to have substantiallysimilar melting temperatures (T_(m)). The second polymer material mayalso constitute any crystalline or glassy polymer. Moreover, the firstpolymer material and the second polymer material may be selected to havethe same, substantially the same, or different gas permeabilities.

Examples of polymers that can be used as the second polymer materialinclude, but are not limited to: polyolefins, polyacetals (orpolyoxymethylenes), polyamides, polyesters, polysulfides, polyvinylalcohols, polyvinyl esters, and polyvinylidenes. Polyolefins include,but are not limited to: polyethylene (including, for example, LDPE,LLDPE, HDPE, UHDPE), polypropylene, polybutylene, polymethylpentane,co-polymers thereof, and blends thereof. Polyamides (nylons) include,but are not limited to: polyamide 6, polyamide 66, Nylon 10,10,polyphthalamide (PPA), co-polymers thereof, and blends thereof.Polyesters include, but are not limited to: polyester terephalthalate,polybutyl terephalthalate, co-polymers thereof, and blends thereof.Polysulfides include, but are not limited to, polyphenyl sulfide,co-polymers thereof, and blends thereof. Polyvinyl alcohols include, butare not limited to: ethylene-vinyl alcohol, co-polymers thereof, andblends thereof. Polyvinyl esters include, but are not limited to,polyvinyl acetate, ethylene vinyl acetate, co-polymers thereof, andblends thereof. Polyvinylidenes include, but are not limited to:fluorinated polyvinylidenes (e.g., polyvinylidene chloride,polyvinylidene fluoride), co-polymers thereof, and blends thereof.Although specific materials for the second polymer material areenumerated, it will be appreciated that alternative polymers,copolymers, and combinations thereof may be used that meet theaforementioned performance criterion for the multilayer film 10.

One or more additives, such as ultraviolet blockers, coloring additives,and nucleating agents may be added to the second polymer material toform the second polymer layer 14. In one example, the additive is afiller used to impart particular properties, e.g., enhanced porosity,upon the second polymer layer 14. Examples of fillers can include, butare not limited to, calcium carbonate (CaCO₃), various kinds of clay,silica (SiO₂), alumina, barium sulfate, sodium carbonate, talc,magnesium sulfate, titanium dioxide, zeolites, aluminum sulfate,cellulose-type powders, diatomaceous earth, magnesium sulfate, magnesiumcarbonate, barium carbonate, kaolin, mica, carbon, calcium oxide,magnesium oxide, aluminum hydroxide, pulp powder, wood powder, cellulosederivatives, polymer particles, chitin, and chitin derivates, and blendsthereof.

Alternatively or additionally, nucleating agents may be added to thesecond polymer material to increase the porosity of the second polymerlayer 14 upon axially orientation. Examples of nucleating agents caninclude, but are not limited to, quinacridone quinone (QQ) and Millad(MD). When polypropylene is nucleated with, for example, QQ, a β-form ofPP is observed. In addition to regular β-form, QQ nucleates PP intoα-form. Additional instances of β-form nucleating agents and formationsare disclosed in U.S. Pat. Nos. 5,134,174; 5,231,126; 5,317,035; and5,594,070; EPO Publication No. 632,095; Japanese Kokai Nos. 7-118429 &9-176352; Chu, F. et al., “Microvoid formation process during theplastic deformation of Beta-form polypropylene”, POLYMER v34 n16, 1994;Chu, F. et al., “Crystal transformation and micropore formation duringuniaxial drawing of Beta-form polypropylene film”, POLYMER v36 n13,1995; Ikeda, N. et al., “NJ-Star NU-100: A Novel Beta-Nucleator forPolypropylene”, Polypropylene & World Congress, Sep. 18-20, 1996; Zhu,W. et al., “A New Polypropylene Microporous Film”, Polymers for AdvancedTechnologies, v7, 1996, each of which is incorporated herein byreference. In one instance, the first polymer material constitutes PEBAXand the second polymer material constitutes polypropylene with one of QQor CaCO₃.

The axially oriented, coextruded multilayer film can have a thickness ofabout 1 μm to about 2500 μm. The thickness of the axially orientedcoextruded first polymer selective layers 12 axially oriented, can varyand be, for example, from about 5 nm to about 1000 nm, from about 10 nmto about 100 nm, or from about 10 nm to about 20 nm. The thickness ofthe axially oriented, coextruded second polymer support layer can beabout 5 nm to about 10 μm, for example about 50 nm to about 1 μm. Insome embodiments, the thickness(es) of the axially oriented, coextrudedfirst polymer selective layers can be such that the total thickness orthe combined thicknesses of all the first polymer selective layers inthe axially oriented, coextruded multilayer film is less than about 1μm. Advantageously, limiting the total thickness or the combinedthicknesses of the at least one first polymer selective layer in theaxially oriented, coextruded multilayer film to less than 1 μm canprovide the multilayer film with a high flux (e.g., at least about 20GPU or at least about 30 GPU) that is suitable for packagingapplications. It will be appreciated that the thicknesses of the axiallyoriented, coextruded first polymer selective layers 12 and the secondpolymer support layers 14 can be readily selected to optimize reductionof the thicknesses first polymer layers 12 without causing structuraldefects in the first polymer layers 12 or the axially orientedmultilayer film.

The axially oriented, coextruded multilayer film 10 can be prepared byinitially coextruding the first polymer material and the second polymermaterial to form the two polymer layers 12, 14. Coextrusion of the firstpolymer material and the second polymer material can yield a large,flexible film 10 or sheet of multilayer structure including alternatingfirst polymer layers 12 and second polymer layers 14. Alternatively, acoextruded multilayer film 10 may be formed that includes only one firstpolymer layer 12 and/or only one second polymer layer 14 (not shown).

A typical multilayer coextrusion apparatus is illustrated in FIGS. 3 and4. The two component, e.g., first polymer material (a) and secondpolymer material (b), coextrusion system consists of two ¾ inch singlescrew extruders each connected by a melt pump to a coextrusionfeedblock. The feedblock for this two component system combines firstpolymeric material (a) and second polymeric material (b) in an (AB)layer configuration. The melt pumps control the two melt streams thatare combined in the feedblock as two parallel layers. By adjusting themelt pump speed, the relative layer thickness, e.g., the thickness ratioof A to B, can be varied. From the feedblock, the melt goes through aseries of multiplying elements. A multiplying element first slices theAB structure vertically, and subsequently spreads the melt horizontally.The flowing streams recombine, doubling the number of layers. Anassembly of n multiplier elements produces an extrudate with the layersequence (AB)^(x) where x is equal to (2)^(n) and n is the number ofmultiplying elements. It is understood by those skilled in the art thatthe number of extruders used to fabricate the structure of the inventionequals the number of components. Thus, a three-component multilayer (ABC. . . ), requires three extruders.

In some embodiments, the coextruded multilayer film 10 can have at least3 alternating layers, but may have more or fewer layers. In one example,the multilayer film 10 of the present invention has 3 layers. Byaltering the relative flow rates or the number of layers 12, 14, whilekeeping the film or sheet thickness constant, the individual layerthickness can be controlled. The coextruded multilayer film 10 or sheetcan have an overall thickness ranging, for example, from about 1 μm toabout 25,000 μm, and any increments therein.

In some aspects, the layers 12, 14 can be coextruded at a temperaturethat is above the melting temperature (T_(m)) of the first polymermaterial and optionally also above the melting temperature (T_(m)) ofthe second polymer material. The coextrusion temperature may be chosensuch that the viscosities of the first and second polymer materials usedto form the first and second polymer layers 12, 14 are as close aspossible to one another to promote uniformly coextruded layers. Thecoextrusion can be performed to render the first and second polymerlayers 12, 14 as thin as possible without inducing tensile failure orotherwise structurally compromising the first polymer layers 12.Coextruding the multilayer film 10 causes the first polymer layers 12 toexhibit a first gas permeability and the second polymer layers 14 toexhibit a second gas permeability. The first and second gaspermeabilities may be the same, substantially the same, or different.

After coextrusion, the coextruded multilayer film 10 is axially orientedor stretched at a temperature below the melting temperatures (T_(m)) ofthe first polymer material or layer 12 and the second polymer materialor layer 14. Axial orientation should be sufficient to reduce the totalthickness of the first polymer layer 12 or the combined thicknesses ofthe first polymer layers to less than about 1 μm in order to potentiallyimprove the flux of the first polymer layers without affecting or whilemaintaining the CO₂/O₂ selectivity of the axially oriented, coextrudedmultilayer film. Axial orientation of the first polymer layers 12 belowthe melting temperatures (T_(m)) of the first polymer layer 12 canresult in strain-induced crystallization, e.g., substantiallycrystalline lamellae, within the microstructure of the first polymermaterial (a) of the first polymer layers 12 that can reduce thepermeability of the first polymer selective layers from a firstpermeability to a second lower permeability. In certain embodiments,axial orientation of the coextruded multilayer film can result in orcause each first polymer layer 12 to crystallize as a high aspect ratiosubstantially crystalline lamellae, which has a reduced permeability.

In contrast, axial orientation of the multilayer film below the meltingtemperature (T_(m)) of the second polymer layers 14 can make the secondpolymer layers more porous or more permeable such that the secondpolymer support layers have a substantially greater (e.g., at leastabout 5 times greater) gas permeability and flux than the first polymerselective layers. In certain embodiments, axially orienting thecoextruded multilayer film 10 decreases the density of the secondpolymer material (b) of the second polymer layers 14. When β-nucleatingadditives are present in the second polymer layer 14, axial orientationof the second polymer material (b) induce α-phase crystallinity in thesecond polymer layer 14, which introduces voids in the second polymerlayer 14, thereby increasing porosity and permeability through thesecond polymer layer 14.

Axial orientation of the multilayer film 10 may be uniaxial or biaxialand may be undertaken at a temperature of from about 23° C. to about120° C., for example, from about 70° C. and about 100° C., depending onthe melting temperatures (T_(m)) of the first polymer material and thesecond polymer material 14. If the multilayer film 10 is biaxiallyoriented, the draw rate may be symmetric or asymmetric. The coextrudedmultilayer film 10 may, for instance, be drawn at strain rates of, forexample, about 50%/min or about 100%/sec to an axially oriented lengthof about 100% to about 400%. The drawing may be constrained orunconstained. In one example, the multilayer film can be simultaneouslybiaxially drawn to draw ratios varying from about 1.8:1.8 to about 5:5at a temperature of between 90° C.-110° C. and a strain rate of about20-500%/s, although other draw ratios may be used.

Axially orienting the multilayer film 10 increases the gas permeabilityof the second polymer layers 14 above the second gas permeability of theaxially oriented first polymer layers 12 such that the lower gaspermeability of the first polymer layers 12 dictates the gaspermeability, selectivity, and flux of the axially oriented, coextrudedmultilayer film 10. In other words, axially orienting the multilayerfilm 10 alters the multilayer film 10 from an unoriented condition inwhich the first and second polymer layers 12, 14 have a first gaspermeability relationship, e.g., the same or different gaspermeabilities, to an axially oriented condition having a second gaspermeability relationship in which the second polymer layers 14 have asubstantially greater gas permeability than the first polymer layers 12.In certain embodiments, axially orienting the multilayer film 10 resultsin a thin, multilayer film that exhibits a CO₂/O₂ selectivity of about 6to about 10, high gas permeability, and a flux of about 30 GPU.

FIG. 4 illustrates a method of preparing the multilayer film 10 whenaxial orientation of the coextruded multilayer film 10 producesstrain-induced crystallization in the first polymer layers 12. When thecoextruded multilayer film 10 is axially oriented in this manner, thestrain on the first polymer material (a) of the first polymer layers 12induces crystallization along and within the first polymer layers 12.The crystallization results in the formation of lamellae within thefirst polymer layers 12 that extend parallel to the plane of the firstpolymer layers 12. The lamellae therefore extend in a direction that isperpendicular or transverse to the gas flow direction through themultilayer film 10. Accordingly, strain-induced crystallization in thefirst polymer layers 12 reduces the gas permeability of the axiallyoriented, coextruded first polymer layers 12 and, thus, the gaspermeability of the axially oriented, coextruded multilayer film 10 isreduced. Although the multilayer film 10 may regain some permeabilityupon elastic relaxation and recovery of the film following axialorientation, residual strain-induced crystallinity within the firstpolymer layers 12 ultimately results in an overall reduced gaspermeability. In some instances, strain-induced crystallization maycause up to about a 3.5× reduction in permeability of the first polymerlayers 12 relative to the permeability of unoriented first polymerlayers 12.

Since it is desirable to maintain high gas permeability in certainapplications, e.g., produce packaging, it is beneficial to remove orreverse the effects of strain-induced crystallization in the axiallyoriented, coextruded first polymer layers 12. Removing or reversing theeffects of strain-induced crystallization in the first polymer layers 12can result in restoring high gas permeability to the first polymerlayers 12 and, thus, restoring high gas permeability and flux in themultilayer film 10. In some embodiments, the effects of strain-inducedcrystallization in the axially oriented first polymer layers 12 can beremoved and/or restored by heat treating and/or annealing the axialoriented multilayer film at temperature and for a time effective toremove at least some of the strain-induced crystallization in the firstpolymer layers 12 without decreasing the selectivity of the firstpolymer layers.

The heat treating or annealing can be conducted at a temperature that ishigher than the melting of the strain-induced crystallinity, e.g., at42° C. for PEBAX. Annealing the multilayer film 10 causes the elongatedlamellae in the first polymer layers 12 to relax along their length,thereby increasing the permeability of the first polymer layers 12 and,thus, increasing the permeability of the multilayer film 10. Thepermeability of the first polymer layers 12 may increase to a level thatis substantially equal to or above the permeability of the first polymerlayers 12 prior to axial orientation to provide the desired gaspermeability, selectivity, and flux for a particular application. Inother words, annealing substantially restores the gas permeability ofthe multilayer film 10.

In some embodiments, the coextruded multilayer film can be heat treatedor annealed prior to axial orientation to substantially increase gasflux (e.g., at least about 50% to about 100%) of the axially oriented,coextruded multilayer film. It was found that annealing a coextrudedmultilayer film comprising a first selective layer of PEBAX and a secondsupport layer of nucleated polypropylene at temperature of about 140° C.for thirty minutes prior to orientation improved the gas flux of theaxially oriented, coextruded multilayer film about 50% to about 100%compared to similar axially oriented, coextruded multilayer films thatwere not annealed prior to axial orientation. This results from improvedβ-polypropylene crystallinity that leads to more and larger pores in thepolypropylene when the multilayer film is axially oriented.

Once the multilayer film 10 is coextruded, axially oriented, and—ifapplicable—annealed, the film 10 may be formed into a number of articlesby, for example, thermoforming, vacuum forming, or pressure forming.Further, through the use of forming dies, the multilayer films 10 may beformed into a variety of useful shapes including profiles, tubes, andthe like. Since the coextruded, axially oriented, multilayer film 10achieves a thin, highly selective construction while being manufacturedin a solventless process, the coextruded, multilayer film 10 of thepresent invention is advantageous over prior multilayer films.

The following examples are for the purpose of illustration only and arenot intended to limit the scope of the claims, which are appendedhereto.

Example 1

In the present example, polymer film layers formed from PEBAX® gradeshaving various composition ratios (vol/vol) including PTMO/PA-12 80/20,70/30, 53/47, 38/62, 25/75, 20/80, and 15/85 were tested for gaspermeability before the layers were axially oriented. FIG. 5 and Table 1illustrate that the addition of PTMO significantly increases thepermeability of the polymer film layer as compared to Nylon-12.Furthermore, it is clear that the selectivity remains constant at about9 in the PEBAX grades having high PTMO content.

TABLE 1 PEBAX grade 2533 3533 4033 5533 6333 7033 7233 Nylon-12 % PTMO80 70 53 38 25 20 15 0 P(CO₂)/ 9.8 9.3 9.32 9.2 7.9 6.2 4.8 4.2 P(O₂)

Referring to FIG. 6, the polymer film layers were uniaxially stretched,thereby producing strong alignment and strain-induced crystallization ofthe PTMO blocks. Confirmation of the oriented lamellar morphology of thepolymer film layers and details of the global orientation were obtainedwith wide angle X-ray scattering (WAXS). FIG. 7 and Table 2 illustratethat axially orientating the polymer films containing PEBAX decreasesthe oxygen permeability of the polymer film. In particular, it is clearthat PEBAX grades containing at least 40% PTMO had up to a 3.5× decreasein O₂ permeability with increasing strains. The CO₂/O₂ selectivity ofthe same PEBAX gradient, however, stayed substantially the same.

TABLE 2 PEBAX Grade 2533 3533 4033 5533 % PTMO 80 70 53 38 P(O₂)decrease 3.5x 3.0x 2.1x 1.8x

The strain-induced crystallization leads to reduced permeability of thepolymer film layers upon orientation. In particular, the WAXS imagesshow two equatorial reflections from orientation and strain-inducedcrystallization of the PTMO blocks. Once the layers were allowed toelastically recover at room temperature by removing the applied stress,the oxygen permeability of each PEBAX grade was calculated as shown inFIG. 8. In particular, oxygen flux J(t) at 0% relative humidity, 1 atm,and 23° C. was measured with a MOCON OX-TRAN 2/20. The permeant gasstream was diluted with nitrogen to achieve a 2% oxygen concentration inorder to avoid exceeding the detector capability of the instrument.Permeability was obtained from the steady flux J₀ according to:P=J ₀ l/pwhere p is the oxygen pressure and l is the film thickness. Two filmsprepared under the same conditions were tested to obtain the averagepermeability. The permeability can be split into the solubility (S) anddiffusivity (D). Usually S and D are extracted from the non-steady stateflux curve.

The strain-induced crystallization caused reduced permeability (up toabout 3.5× in some cases) in oriented PEBAX films relative to thepermeability of the original, unoriented PEBAX films. PEBAX grades withmore PTMO showed a greater effect of strain-induced crystallinity. WAXSimages illustrating the residual crystallinity in the PEBAX layers areshown in FIG. 6.

In order to reverse the reduced permeability caused by thestrain-induced crystallization, the axially oriented polmyer film layerswere annealed. More specifically, the polymer film layers were heattreated at 60° C. for 30 minutes in order to reverse the strain-inducedcrystallization. WAXS patterns of the polymer film layers prior to andfollowing annealing are shown in FIG. 10. Using commercial instrumentsfrom MOCON (D. J. Sekelik, E. V. Stepanov, S. Nazarenko, D. Schiraldi,A. Hiltner, E. Baer, J. Polym. Sci. Pt. B-Polym. Phys. 37, 847-857(1999)), the oxygen permeability was then measured on the polymer films.As shown in FIG. 9, after annealing the O₂ permeability of 200% strainsamples of the polymer films returned to values similar to theunoriented controls.

Example 2

In this example shown in FIGS. 11-13, multilayer films included threealternating layers of PEBAX and PP+CO₃. In one instance, the multilayerfilm included two PEBAX layers and one layer of PP+CO₃. In anotherinstance, the multilayer film included one PEBAX layer and two layers ofPP+CO₃. The co-extruded films had PEBAX/(PP+CaCO₃) volume compositionsof 10/90 or 30/70. The multilayer films were coextruded, uniaxiallyoriented at 23° C., 50%/min to 250%. Oxygen flux of the uniaxiallyoriented films is illustrated in FIG. 11 with FIG. 12 illustrating theeffect temperature has on axial orientation of the multilayer films.FIG. 12 makes clear that stretching or axially orienting the multilayerfilms increases gas flux when conducted above the melting temperature inwhich strain-induced crystallinity is induced, e.g., T_(m)=42° C. At 60°C., however, it appears that gas flux through the multilayer filmsdecreases due to reduced pore formation and the collapse of pores in thePP layers. That being said, the membranes appeared to have similarpermeation values and trends regardless of the film structure, e.g.,PEBAX layers on the outside or inside of the multilayer film.

Referring to FIG. 13, after annealing, the oxygen flux of all samplesroughly doubled by reducing or eliminating strain-inducedcrystallization within the PEBAX layers. The PEBAX thickness did notappear to affect the amount of flux increase.

Example 3

In this example shown in FIGS. 14-15, multilayer films included threealternating layers of PEBAX and PP+QQ. In one instance, the multilayerfilm included two PEBAX layers and one layer of PP+QQ. In anotherinstance, the multilayer film included one PEBAX layer and two layers ofPP+QQ. The multilayer films had PEBAX/(PP+QQ) volume compositions of10/90 and 30/70. The multilayer films were coextruded, biaxiallyoriented at 100° C., 100%/min, and 2×2. Oxygen flux of the biaxiallyoriented films is illustrated in FIG. 14. The membranes appeared to havesimilar permeation values and trends regardless of the film structure.After annealing, the oxygen flux of all samples remained roughlyconstant (FIG. 15). The PEBAX thickness and film structure did notappear to affect the annealing results.

Example 4

In this example shown in FIGS. 16-18, multilayer films included threealternating layers of PEBAX and β-PP. In one instance, the multilayerfilm included two PEBAX layers and one layer of β-PP. In anotherinstance, the multilayer film included one PEBAX layer and two layers ofβ-PP. The multilayer films had PEBAX/β-PP volume compositions of 10/90and 30/70. The multilayer films were coextruded, biaxially oriented at100° C., and annealed. Oxygen flux of the biaxially oriented films priorto annealing is illustrated in FIG. 16. The membranes appeared to havesimilar permeation values and trends regardless of the film structure.

Table 3 illustrates that the multilayer films showed much lowerpermeability when uniaxially stretched compared to biaxial stretching.The 10/90, three layer films having a 4 mils thickness were stretcheduniaxially to 200% strain at various temperatures and strain rates. Thetest indicated that low temperatures and high strain rates produce filmsthat appear to have higher porosity.

TABLE 3 Biaxially 50° C. 50° C. 70° C. 70° C. 100° C. Oriented 50%/min1000%/min 50%/min 1000%/min 50%/min 100%/s P(O₂) 13 11.9 5.3 9.8 1.611.9 PEBAX Thickness (μm) 5 4.6 3.6 4 3.9 4.6

Referring to FIG. 17, post-orientation annealing indicated that theoxygen flux of all samples remained roughly constant as the PEBAX layersdid not appear to strain crystallize under the orientation conditionsused. In contrast, pre-orientation annealing at 140° C. for 30 minutessignificantly increased the oxygen flux (about 50-100%) due to theformation of more and/or larger pores in the β-PP layers (FIG. 18). ThePEBAX thickness and film structure did not appear to affect theannealing results. In any case, it appears that orientation at highstrain rates and under low temperatures produces desirable flux valuesfor PEBAX/β-PP multilayer films.

While a preferred embodiment of the invention has been illustrated anddescribed, it shall be understood that the invention is not limited tothis embodiment. Numerous modifications, changes and variations will beobvious for those skilled in the art, without departing from the scopeof the invention as described by the appended claims. All patents,publications, and references cited herein are incorporated by referencein their entirety.

Having described the invention, the following is claimed:
 1. A foodpackaging membrane comprising: an axially oriented, coextrudedmultilayer film that has a flux of at least about 10 GPU, the axiallyoriented, coextruded multilayer film including at least one axiallyoriented, coextruded first polymer layer of a first polymer material andat least one axially oriented, coextruded second polymer layer of asecond polymer material, the at least one axially oriented, coextrudedfirst polymer layer having a first permeability (P₁) prior to axialorientation and a second permeability (P₂) after axially orientationless than or equal to the first permeability (P₁), the at least oneaxially oriented, coextruded second polymer layer having a firstpermeability (P_(1a)) prior to axial orientation and a secondpermeability (P_(2a)) after axial orientation that is substantiallygreater than the first permeability (P_(1a)) and the second permeability(P₂), the axially oriented, coextruded first polymer layer having athickness and the combined thicknesses of all the axially oriented,coextruded first polymer layers of the axially oriented, coextrudedmultilayer film being less than about 1 μm.
 2. The food packagingmembrane of claim 1, wherein the axially oriented, coextruded multilayerfilm having a CO₂/O₂ selectivity of at least about
 4. 3. The foodpackaging membrane of claim 1, wherein the axially oriented, coextrudedfirst polymer layers having a CO₂/O₂ selectivity of at least about
 4. 4.The food packaging membrane of claim 1, wherein the axially oriented,coextruded multilayer film having a flux of at least about 30 GPU. 5.The food packaging membrane of claim 1, wherein the first polymermaterial comprises a poly(ether block amide).
 6. The food packagingmembrane of claim 5, wherein the first polymer material comprisespoly(ether block amide) that includes from about 15% to about 80% of apolyether by molecular weight.
 7. The food packaging membrane of claim1, wherein the second material comprises polypropylene.
 8. The foodpackaging membrane of claim 7, wherein the second polymer materialfurther comprises CaCO₃ or a beta-nucleation agent.
 9. The foodpackaging membrane of claim 1, wherein the axially oriented, coextrudedmultilayer film comprises a plurality of axially oriented, coextrudedalternating first polymer layers and second polymer layers.
 10. A methodof fabricating a food packaging membrane comprising: coextruding a firstpolymer material and a second polymer material to form a multilayer filmthat includes at least one coextruded first polymer layer and at leastone coextruded second polymer layer, the at least one first polymerlayer having a first permeability (P₁); and axially orienting thecoextruded multilayer film, the at least one axially oriented,coextruded first polymer layer having a second permeability (P₂) afteraxial orientation less than or equal to the first permeability (P₁), theat least one axially oriented, coextruded second polymer layer having afirst permeability (P_(1a)) prior to axial orientation and a secondpermeability (P_(2a)) after axial orientation that is substantiallygreater than the first permeability (P_(1a)) and the second permeability(P₂), wherein the axially oriented, coextruded multilayer film has aflux of at least about 10 GPU, the axially oriented, coextruded firstpolymer layer having a thickness and the combined thicknesses of all theaxially oriented, coextruded first polymer layers of the axiallyoriented, coextruded multilayer film being less than about 1 μm.
 11. Themethod of claim 10, wherein the multilayer film is axially oriented at atemperature below the melting temperature (T_(m)) of the second polymermaterial.
 12. The method of claim 10, wherein the multilayer film isuniaxially stretched.
 13. The method of claim 10, wherein the at leastone first polymer layer having a CO₂/O₂ selectivity of at least about 4and the axially oriented, coextruded multilayer film having a CO₂/O₂selectivity of at least about
 4. 14. The method of claim 10, wherein thefirst polymer material comprises a poly(ether block amide).
 15. Themethod of claim 14, wherein the first polymer material comprisespoly(ether block amide) that includes from about 15% to about 80% of apolyether by volume.
 16. The method of claim 10, wherein the secondpolymer material comprises polypropylene.
 17. The method of claim 16,wherein the second polymer material further comprises CaCO₃ or abeta-nucleation agent.
 18. The method of claim 10, wherein the axiallyoriented, coextruded multilayer film comprises a plurality of axiallyoriented, coextruded alternating first polymer layers and second polymerlayers.
 19. The method of claim 10, wherein the multilayer film isformed in a solventless process.
 20. The method of claim 10, wherein themultilayer film is axially stretched from about 100% to about 400%. 21.The method of claim 10, further comprising heat treating the multilayerfilm at a temperature and for a time to increase the gas permeability ofthe at least one axially oriented, coextruded first polymer layer. 22.The method of claim 21, wherein axially orienting the at least one firstpolymer layer causes strain-induced crystallization in the at least onefirst polymer layer and the heat treating at least partially reversingcrystallization in the at least one first polymer layer to increase thegas permeability of the at least one first polymer layer.